This invention relates to a matrix of coaxial transmission lines, particularly a Butler matrix for the distribution of electromagnetic energy from one of a plurality of input ports to a plurality of output ports and, more particularly, to a set of coaxial transmission lines constructed in a unitary assembly wherein paired coupling devices formed of closely spaced center conductors of adjacent coaxial lines including a crossed configuration of the center conductors provide for in-plane crossing of power from one transmission line to another transmission line.
In the processing of electromagnetic signals, it is frequently advantageous to distribute and combine algebraically signals propagating in a set of waveguides. A common example of such combination is found in the feeding of antenna elements in an array antenna in which each element is fed microwave energy via a coaxial transmission line. As is well known, the contributions of electromagnetic energy applied to each of the antenna elements radiate as waves, and combine to form a beam upon suitable phasing of the waves radiated by the respective elements. The difference in phase among waves of the various elements, sometimes referred to as a phase taper or phase slope, can be selected to adjust a direction of radiation of the beam from the antenna.
One form of microwave distribution system for distributing the electromagnetic energy among the antenna elements is composed of a set of lines for transmission of electromagnetic energy interconnected to form a matrix of paths for the conduction of electromagnetic energy, the composite transmission-line structure being known as a Butler matrix. The Butler matrix is well known and may be used for coupling, by way of example, a set of four input ports to a set of four output ports, a set of eight input ports to a set of eight output ports, or other number of ports such as sixteen input ports to sixteen output ports. Assuming by way of further example that the output ports are connected to an array antenna, and that the input ports are connected via a selector switch to a transmitter, energization of any one of the input ports with electromagnetic power provides for a uniform distribution of the electromagnetic power among the full set of output ports to provide for a radiated beam from the antenna. The direction of the beam relative to the array of antenna elements differs with each selected one of the input ports. Thereby, by operation of the selector switch, a beam may be generated in any desired one of a set of possible directions. The Butler matrix is reciprocal in operation so that a receiving beam of radiation can be outputted at any one of the input ports for coupling by the selector switch to a receiver.
A Butler matrix is composed of numerous 3 dB (decibels) couplers interconnecting transmission lines whereby power in one transmission line can be distributed equally between one transmission line and a second transmission line. A 90 degree phase shift is introduced at the coupler between waves carrying each half of the power. Therefore, various phase relationships exist among waves traveling in the various transmission lines. In order to provide for a desired phase taper at the output ports for forming a beam on transmission, and in order to sum together the contributions from various antenna elements during reception of an incoming electromagnetic wave, additional phase shifters are connected into the waveguides. A further aspect in the construction of a Butler matrix is the presence of numerous crossovers in which one transmission line is provided with twists and turns to cross over another transmission line, thereby to allow interconnection and coupling of signals between various combinations of the transmission lines.
A problem arises in the construction of a Butler matrix, or other matrix of transmission lines employed for the algebraic combination of electromagnetic waves, in that the manufacture of an assembly of transmission lines with twists and turns to effect a crossover is difficult. Furthermore, in the case of a matrix interconnecting many input ports with many output ports, there are crossings of transmission lines above other crossed over transmission lines resulting in a microwave structure of highly irregular shape and excessively large size which is difficult to incorporate into a microwave system.